I would say if he wanted to bring a guest you could give him that option but I wouldn't be paying for that person. That's just me though

I totally agree. He should be allowed to bring a friend or something b/c he may not know very many people in your group and might want to have someone to hang out with when he not officiating your ceremony. I also agree that you don't have to pay for the guest.

y dont you have your rabbi marry you here, and just have a symbolic ceremony, noone will know the difference because you can even find a rabbi there to marry you and your guests will think that this is the actual wedding, but it saves you having to fly anyone out and deal with any other mexican legal restrictions

Thank you all so much for all of your responses--- I really appreciate them. I think I am goign to still fly him down, but not pay for a guest. He knows about ten of the guests, so it shouldn't be an issue in that way.